The U.S. Department of Transportation (DOT) and the U.S. Environmental Protection Agency (EPA) have established U.S. federal rules that set national greenhouse gas emission standards. Beginning with 2012 model year vehicles, automobile manufacturers are required to reduce fleet-wide greenhouse gas emissions by approximately five percent every year. Included in the requirements, for example, the new standards decree that new passenger cars, light-duty trucks, and medium-duty passenger vehicles must have an estimated combined average emissions level no greater than 250 grams of carbon dioxide (CO2) per mile in vehicle model year 2016.
Catalytic converters are used in internal combustion engines to reduce noxious exhaust emissions arising when fuel is burned as part of the combustion cycle. Significant among such emissions are carbon monoxide and nitric oxide. These gases are dangerous to health but can be converted to less noxious gases by oxidation respectively to carbon dioxide and nitrogen/oxygen. Other noxious gaseous emission products, including unburned hydrocarbons, can also be converted either by oxidation or reduction to less noxious forms. The conversion processes can be effected or accelerated if they are performed at high temperature and in the presence of a suitable catalyst being matched to the particular noxious emission gas that is to be processed and converted to a benign gaseous form. For example, typical catalysts for the conversion of carbon monoxide to carbon dioxide are finely divided platinum and palladium, while a typical catalyst for the conversion of nitric oxide to nitrogen and oxygen is finely divided rhodium.
A catalytic converter may take any of a number of forms. Typical of these is a cylindrical substrate of ceramic material generally called a brick. The brick has a honeycomb structure in which a number of small area passages or cells extend the length of the brick, the passages being separated by walls. There are typically from 400 to 900 cells per square inch of cross-sectional area of the substrate unit and the walls are typically in the range 0.006 to 0.008 inches in thickness. The ceramic substrate is formed in an extrusion process in which green ceramic material is extruded through an appropriately shaped die and units are cut successively from the extrusion, the units being then cut into bricks which are shorter than a unit. The areal shape of the passages may be whatever is convenient for contributing to the overall strength of the brick while presenting a large contact area at which the flowing exhaust gases can interact with a hot catalyst.
The interiors of the passages in the bricks are wash-coated with a layer of the particular catalyst material. The wash-coating is prepared by generating a suspension of the finely divided catalyst in a ceramic paste or slurry, the ceramic slurry being to obtain adhesion of the wash-coated layer to the walls of the ceramic substrate. As an alternative to wash-coating to place catalyst materials on the substrate surfaces, the substrate material itself may contain a catalyst component so that that the extrusion presents catalyst material at the internal surfaces bounding the substrate passages or cells.
A catalytic converter may have a series of such bricks, each having a different catalyst layer depending on the particular noxious emission to be neutralized. Catalytic converter bricks may be made of materials other than fired ceramic, such as stainless steel. In addition, ceramic substrates may have different forms of honeycombed passages than those described above. For example, substrate cells can be hexagonal or triangular in section. In addition, if desired for optimizing strength and low thermal capacity or for other purposes, some of the extruded honeycomb walls can be formed so as to be thicker than other of the walls or formed so that there is some variety in the shape and size of honeycomb cells. Junctions between adjacent interior cell walls can be sharp angled or can present curved profiles.
The wash-coated ceramic honeycomb brick is wrapped in a ceramic fibre expansion blanket. A stamped metal casing transitions between the parts of the exhaust pipe fore and aft of the catalytic converter and encompasses the blanket wrapped brick. The casing is made up of two parts which are welded to seal the brick in place. The expansion blanket provides a buffer between the casing and the brick to accommodate their dissimilar thermal expansion coefficients. The sheet metal casing expands many times more than the ceramic at a given temperature increase and if the two materials were bonded together or in direct contact with each other, destructive stresses would be experienced at the interface of the two materials. The blanket also dampens vibrations from the exhaust system that might otherwise damage the brittle ceramic.
In use, the encased bricks are mounted in the vehicle exhaust line to receive exhaust gases from the engine and to pass them to the vehicle tail pipe. The passage of exhaust gases through the catalytic converter heats the brick to promote catalyst activated processes where the flowing gases contact the catalyst layer. Especially when the vehicle engine is being run at optimal operating temperature and when there is substantial throughput of exhaust gases, such converters operate substantially to reduce the presence of noxious gaseous emissions entering the atmosphere. It is known, however, that such converters have shortcomings at start-up when the interior of the brick is not at high temperature and during idling which may occur frequently during city driving or when stopping for a coffee at Tim Hortons. The radial transmission of heat in this and other forms of catalytic converter occurs by a combination of convection, conduction and radiation. The various heating mechanism have different effects at different converter temperatures. In particular, at low temperatures before the converter has reached optimal operating temperature, heat transfer is predominantly by convection of gases and by conduction along and through the interconnected ceramic walls. At normal operating temperature, heat transfer is predominantly by radiation generally from the core of the converter towards its periphery.
U.S. Pat. No. 8,309,032 (Plati et al.), which is herein incorporated by reference in its entirety, describes a particular form of catalytic converter component for use in an exhaust system of an internal combustion engine. The component includes a housing having a gas inlet and a gas outlet, and catalytic substrate material filling the housing. The substrate material is divided into zones that are separated from one another by an insulating barrier, the zones defining flow passages connecting the inlet and outlet for the passage of exhaust gases. In certain operating regimes, this configuration results in a reduction in heat transfer between a core zone and a surrounding zone of the component. Thus, at start up as a majority of relatively cool gases flow though a central part of the converter brick, heat tending to transfer radially outwardly from the core zone by convection and conduction is inhibited by the presence of the insulating barrier. The core of the converter component thus heats more rapidly from a cold start compared with a conventional catalytic converter without the thermal insulating barrier. When the converter component is operating at an optimal operating temperature, any heat transfer is predominantly by radiation which is affected by the insulating barrier to a much reduced extent.
A reverse effect occurs when the engine is at its optimal operating temperature, but the vehicle experiences a period of idling. At this point, the reduced level of exhaust gases passing into the converter start to localize along the converter core and also start to cool the converter down. The presence of the thermal insulating barrier means that the temporary cooling effect is localized in the core zone and is not rapidly or significantly transferred to the radially outer zone of the ceramic brick.
The Plati et al. structure promises significant improvements in lowering emissions and improving fuel mileage and precious metal catalyst savings. In particular, it means that ceramic substrates having of the order of 400 cells per square inch can achieve low emissions which, in the absence of the thermal insulation barrier, would require a substrate having of the order of 900 cells per square inch loaded with precious metal catalyst. However, the placement of an insulating barrier within a catalytic converter component presents difficult manufacturing issues.